Method for producing silanes

ABSTRACT

The invention relates to a method of producing silane or hydrochlorosilanes by disproportionation of a higher chlorinated hydrochlorosilane or a mixture of said hydrochlorosilanes in the presence of a catalyst. Before the catalyst is used, it is a) washed with hyper-pure water in one or several steps; b) transferred in the water-moist state to the reactor in which the disproportionation is to proceed; c) treated in the reactor with boiling methanol or rinsed with anhydrous methanol; and d) the methanol is removed from the catalyst by evacuation and/or stripping with inert gas. The invention further relates to a method for treating disproportionation catalysts.

The present invention relates to a method for producing silane orhydrochlorosilanes by disproportionation of higher chlorinatedhydrochlorosilanes or hydrochlorosilane mixtures in the presence of apretreated catalyst, as well as a method for treating the catalyst.

SiH₄ is an excellently suitable starting material, from which by thermaldecomposition, if necessary after further purification, very puresilicon in semiconductor quality can be deposited. The need forhyper-pure silicon is strongly increasing and thus also the need forpure silane the excellent suitability of which for the manufacture ofhyper-pure silicon is more and more recognized and used.

The manufacture of silane from trichlorosilane by disproportionation isparticularly advantageous in economic terms.

The manufacture of dichlorosilane and silane from hydrochlorosilanes bydisproportionation is carried out essentially in accordance with thegross equationsSiHCl₃→SiCl₄+SiH₂Cl₂  (1a)2SiH₂Cl₂→SiCl₄+SiH₄  (1b)4SiHCl₃→3SiCl₄+SiH₄  (2)

In order to allow the rapid manufacture of silane in this manner also atlow temperatures and without a formation of decomposition products, thepresence of catalysts is helpful. Basic catalysts are particularlyproven, among them amines and amine derivatives, e.g. amine salts, acidamides, nitrites, nitrogen-containing heterocyclic compounds and othernitrogen-containing substances, must be emphasized.

So it is known that amines, particularly tertiary amines and theirhydrochlorides and quaternary ammonium chlorides, both in liquid (DE 3500 318 A1) as well as in solid form, e.g. bound to solid carriers (DE 3311 650 C2), accelerate as catalysts the disproportionation of thetrichlorosilane in an economically advantageous manner. Amines bound tosolid carriers are employed preferably because this way the catalyst canbe separated easily and it can be avoided that polluting amines arecarried in during the reacting gaseous-liquid silane-chlorosilane phase.

For this reason and the facilitation of the procedure related to it,only solid, formed amines, either fixed to carriers or in networkpolymers, are used as catalysts in technical practice.

As a rule, the disproportionation of trichlorosilane is carried out inseveral steps, for example in two steps. It was already described,however, to carry out the disproportionation in one step according tothe principle of reactive distillation. The reactive distillation ischaracterized by combining reaction and distillative separation in oneapparatus, particularly a column. Due to the continuous distillativeremoval of the easiest boiling constituent in each space element anoptimum gradient is always maintained between the balance and the actualcontent of easier boiling constituents and/or easiest boilingconstituent, resulting in a maximum reaction velocity (DE 198 60 146A1).

The advantages of the reactive rectification can be exploitedparticularly by combining it with the catalysis on solids. This isachieved in that the disproportionation of trichlorosilane to silicontetrachloride and silane is carried out in a column, the fillings ofwhich (tower packing, built-ins, etc.) which enable the exchange ofmaterials, are linked with the catalytically active solids.

Because of the fixation of the solid catalyst in the reaction apparatusit is essential to employ it in hyper-pure form and particularly freefrom water. The high purity is a precondition for preventing the productfrom being polluted by the catalyst, the absence of water reduces theproblem of hydrolysis of the chlorosilanes on the catalyst and theproblems related to the hydrolysis products HCl (corrosion,neutralisation of aminic catalyst functions) and silicic acid and/orsubstances similar to silicic acid (deposits on the catalyst).

Hydrochlorosilanes are decomposed by water forming oligomeric siloxanes,trichlorosilane for example according toHSiCl₃+1.5H₂O→1/n(HSiO_(1.5))n+3HCl  (3)

In a dry mass of 4000 kg of catalyst for example, this corresponds tothe quantities of water and hydrolysis products specified in the tablebelow, depending on the moisture contents of the catalyst:

Moisture m (H₂O) m (HSiO_(1.5)) m (HCl) Standard liter [%] [kg] [kg][kg] HCL 0.05 2 4 8 179 0.1 4 8 16 359 0.3 12 24 48 1067 1 40 79 1603586 2 80 157 320 7172 2.5 100 196 400 8966

It is therefore highly required to minimize the hydrolysis ofhydrochlorosilanes by the water residues on the catalyst.

The purification and pretreatment of the catalyst for thedisproportionation of the hydrochlorosilanes has been paid littleattention by now, so that the potential of catalytical activity couldnot been used to the full extent.

In fact, EP 206 621 A1 describes the drying of the catalyst by anintermittent displacement of water by ethanol and toluol, with thecatalyst being suspended in the dried solvents and subsequentlyseparated from the solvent by decantation. The remaining moisturecontents are given with <0.5%. However, neither the removal ofimpurities nor the question of how to integrate this procedure into thetechnical catalysis process and the transfer of the catalyst into thereactor is considered.

In the JPL study “Development of a polysilicon process based on chemicalvapor deposition, Final report (1979–1982); Hemlock SemiconductorCorporation” (DOE/JPL 955533-83, p. 26), the single methods displacementby solvents, vacuum drying, drying in the oven, drying in the desiccatorand rinsing with inert-gas are mentioned, however not further specified,as pretreatment variants for the drying of catalyst for thedisproportionation. Despite desiccator drying all methods were suitablefor pretreatment, however neither the removal of water-solubleimpurities and the effective remaining moisture contents, nor theadaptation of the conditioning to the application of the catalyst in thereactor is paid any attention.

In Ind. Eng. Chent. Res. 1988, 27, 1600–1606, thermal drying of catalystfor disproportionation at 80° C. in vacuum conditions is described as anefficient drying method, however without quantifying the remainingmoisture contents or considering the problem impurities.

It is common for all these procedures, that in the end the pretreatedcatalyst needs to be transferred to the reaction place, i.e. into thereactor where the disproportionation will be carried out. During thistransfer, however, the catalyst again absorbs moisture and airparticles, thus being contaminated anew. In addition to this, it turnedout difficult, or even impossible, to transfer a dry catalyst in theusual commercial spherical form into the containers (pockets, gussets,tubes of the above mentioned fillings, tower packings, built-ins, etc.)where it is supposed to be filled in, in order to enable it being fixedin the reactor (preferably in a reactive rectification column). Thereason for this is that the dry spherically formed catalyst particlesbuild up an electrostatic charge when being poured in, and fly apart dueto their light weight, thus making it impossible to fill them into theintended spaces.

Therefore the task was to provide a method that allows a pretreatment ofsolid catalysts for disproportionation of hydrochlorosilanes prior totheir first contact with hydrochlorosilanes such that the moisturecontents and the contents of organic compounds, particularly those thatare reactive towards hydrochlorosilanes, and the contents of compoundsthat are soluble in hydrochlorosilanes, is minimized, whereby themolecular and macroscopic structure and solidity of the catalyst aremaintained.

Subject-matter of the invention is therefore a method for themanufacture of compounds according to the formula(H)_(x)Si(Cl)_(y)  (I),wherein

-   -   x is 2, 3 or 4, and    -   y is 0, 1 or 2, and    -   x+y=4,        by disproportionation of a hydrochlorosilane of the formula        (H)_(a)Si(Cl)_(b)  (II),        wherein    -   a is 1, 2 or 3, and    -   b is 1, 2 or 3, and    -   a+b=4 and    -   b>y,        or a mixture of such hydrochlorosilane in the presence of a        catalyst, characterized in that the catalyst prior to use    -   a) is washed with hyper-pure water in one or several steps,    -   b) is transferred in a water-moist state into the reactor where        the disproportionation shall be carried out, and    -   c) is either treated with boiling methanol or rinsed with        anhydrous methanol in the reactor, and    -   d) the methanol is removed from the catalyst by evacuation        and/or by stripping with inert gas.

So, according to the invention, hydrochlorosilanes with a lower numberof chlorine substituents or silane starting from higher chlorinatedhydrochlorosilanes is obtained, whereby due to the nature of thedisproportionation reaction also a higher-chlorinated coupled/couplingproduct occurs, which can be defined as educt, however, that reacts tothe desired products in this sense.

The method according to the invention can be carried out discontinuouslyor continuously. A continuous reaction process is preferred.

The method according to the invention is particularly useful for themanufacture of silane by disproportionation of trichlorosilane,dichlorosilane, or a mixture thereof, preferably trichlorosilane.

It is also possible, for example, to obtain dichlorosilane bydisproportionation of trichlorosilane.

Trichlorosilane and dichlorosilane can be employed in this as puresubstances or mixtures with each other or also as mixtures with silicontetrachloride and/or monochlorosilane.

Preferably the disproportionation method according to the invention iscarried out according to the principle of reactive distillation.

Suitable catalytically active solids are known and specified, forexample, in DE-OS-2 507 864. Such suitable solids, for example, aresolids carrying amino groups or alkyleneamino groups on a structure ofpolystyrene, cross-linked with divinylbenzole. Examples of amino groupsor alkylenamino groups are for instance: Dimethylamino, diethylamino,ethyhnethylamino, di-n-propylamino, di-iso-propylamino,di-2-chlorethylanino, di-2-chlorpropylamino groups and the respectivelysubstituted alkyleneamino groups and the respective hydrochlorides, orthe trialkylammonium groups formed from them by methylation, ethylation,propylation, butylation, hydroxyethylation or benzylation with chlorideas counterion. Of course, in the case of quaternary ammonia salts orprotonized ammonia salts also catalytically active solids with otheranions, e.g. hydroxide, sulphate, bi-sulphate, bicarbonate etc. can beintroduced into the method according to the invention, a transformationinto the chloride form, however, is inevitable under the reactionconditions in the course of time, this applies also to organic hydroxygroups. Therefore, ammonia salts containing chloride as counterion arepreferred.

Also those solids are suitable as catalytically active solids whichconsist of a structure of polyacrylic acid, particularly apolyacrylamide structure, that has bound, for example,trialkylbenzylammonium via an alkyl group.

Another group of catalytically active solids suitable for the methodaccording to the invention are for example, solids carrying sulphonategroups on a structure of polystyrene, cross-linked with divinylbenzole,which are opposed by tertiary or quaternary ammonium groups as cations.

As a rule, macroporous or mesoporous ion exchangers are more suitablethan gel resins. Further suitable catalytically active solids are, forexample, solids carrying organic amino groups of the above type, e.g.solids with a 3-dimethyl-aminopropyl-siloxy group, on a solid inorganicstructure like silicic acid or zeolite (U.S. Pat. No. 4,701,430).Suitable catalytically active solids are usually employed in form ofpowdered or spherical contacts.

The preferred resins chosen are polystyrene resins, that arecross-linked with divinylbenzole, with tertiary amine groups in the sidechain, either in form of gel, or more preferred as macroporous resins,because this catalyst type fulfils excellently the requirement ofthermal and chemical stability and high activity.

A number of suitable catalytically active solids is commerciallyavailable.

According to the invention the catalyst is first washed with hyper-purewater (step a)).

Preferably washing is carried out at temperatures from 60 to 90° C.

By washing the catalyst with hyper-pure water water-soluble impuritiesare removed. Washing can be carried out in one or several steps, with atotal contact period of water and catalyst between 0.5 and 50 hours,preferably between 1 and 24 hours.

The ratio of volumes of water to catalyst in each washing step ispreferably 1:1 to 10:1.

Washing, for example, can be carried out in a way that the catalyst issuspended in the washing water or that the washing water runs in a freeor forced flow through the catalyst.

If the catalyst is suspended in the washing water, the adherent watercan be separated mechanically at the end of the washing, for example bydecanting, filtering or centrifuging.

If washing is carried out in several steps, an inorganic alkalinesolution, e.g. soda lye, potash lye or ammonium hydroxide solution, canbe added to the washing water in the first washing step. For example,the concentration of the alkaline lye in the water can be 0.1 to 20weight percent.

According to the invention, after washing the water-moist catalyst istransferred into the reactor where the disproportionation reaction shallbe carried out (step b)).

The catalyst can be easily handled in this form and shows only a lowsensitivity towards the influence of the air.

Step b), for example, can comprise several sub-steps, during which thewater-moist catalyst is first filled into containers with a volume ofapprox. 1–50 ml, e.g. pockets, gussets or tubes in the fillings, towerpackings, built-ins, preferably made of a metallic material, which aresuitable for a good gas-liquid exchange. As a rule, these containerscontaining the disproportionation catalyst are sealed in the nextsub-step, e.g. by welding, soldering, mechanical pressing, and areafterwards assembled to elements to be inserted into the reactor, i.e.preferably the reactive rectification column. It is also possible tojoin the fillings or built-ins containing the catalyst in the column toform elements or packages.

In the following step c), the catalyst contained in the reactor isrinsed with anhydrous methanol and/or treated with boiling methanol.

The ratio of volumes of methanol to catalyst while the catalyst in thereactor is treated with methanol is preferably 0.1:1 to 1:0.3.

Preferably the methanol treatment is carried out twice with freshlyintroduced methanol, particularly preferred 2 to 5 times.

Preferably such treatment of the catalyst in the reactor is carried outwith boiling methanol by heating the methanol at the lower-most end ofthe reactor until it boils, and allowing it to condense at the upper endof the reactor. After the treatment the methanol is removed from thelower part of the reactor. The methanol removes soluble impurities.

If the treatment is carried out with boiling methanol, preferably apressure of 1–3 bar is adjusted during the treatment in the reactor. Dueto the adjusted pressure the temperature of the boiling methanol isadjusted to approx. 64° to approx. 90° C. Between the individual stepsof the methanol treatment preferably inert gas or hydrogen is introducedinto the reactor, which will displaced again from the reactor in thenext step of the treatment.

Each of the steps of the treatment with boiling methanol lasts, forexample, 0.5 to 50 hours, preferably 1–24 hours.

It is preferred to remove a small portion, e.g. 1–10% of the employedmaterial, at the upper end of the reactor in each round together withthe air that is displaced from the reactor or the displaced inert gas orlow boiling substances driven out of the catalyst.

Less preferably, but also possible, step c) can be combined with washingthe catalyst with ethanol propanol and/or isopropanol.

After the catalyst was treated with methanol, this is followed accordingto the invention by the removal of the methanol from the catalyst byevacuating it and/or stripping with inert gas (step d)).

Preferably the methanol is allowed to drop off thoroughly prior toevacuation and/or stripping with inert gas, so that only the residualmethanol remaining in the catalyst and the fillings and built-ins and inthe reactor needs to be removed.

The methanol is preferably removed at temperatures from 60 to 90° C.

Inert gases in meaning applicable here are, for example, nitrogen, noblegases and hydrogen. The use of hydrogen for stripping as well as forpressure compensation after the evacuating steps is preferred in this.The inert gas is preferably heated to the desired temperature of 60–90°C. prior to its entry into the reactor. The flow of the inert gasthrough the reactor can be from the top to the bottom and from thebottom to the top.

The evacuation is reasonably carried out at changing pressures, ideallyimmediately after the last methanol treatment step when the reactor isstill heated up. As the reactor together with the introduced catalystcools down in changing-pressure mode, it is useful in case of severalevacuation steps, to heat up the reactor from time to time to approx. 64to 90° C. by introducing heated inert gas.

It is possible, but not required, after the treatment with methanol(step c)) or after the removal of the methanol from the catalyst (stepd)), to subject the catalyst to a chemical drying procedure. This can beachieved, for example, by adding thionylchloride or phosgen to thecatalyst that was pre-treated in the reactor according to the invention.The residual water reacts with these chemicals without forming solidoxygenic products, actually volatile oxides CO₂ and SO₂ are formed, thusminimizing the risk of obstruction of pores during the subsequentcontact with chlorosilanes.

However, if these water-destroying chemicals are used, the resultinggaseous reaction products CO₂ and/or SO₂ and the unreacted reactantsmust again be removed subsequently by evacuation and/or by strippingwith inert gas.

Subject-matter of the invention is furthermore a method for thetreatment of a catalyst for disproportionation of hydrochlorosilanes,characterized in that the catalyst

-   a) is washed with hyper-pure water in one or several steps,-   b) is transferred in a water-moist state into the reactor where the    disproportionation shall he carried out, and-   c) is either treated with boiling methanol or rinsed with methanol    in the reactor, and-   d) the methanol is removed from the catalyst by evacuation and/or by    stripping with inert gas.

The advantageous and preferred embodiments of this method are inaccordance with what was specified above.

The method according to the invention can be applied, for example, inprocesses for the manufacture of dichlorosilane and silane and as asub-step in processes for the manufacture of hyper-pure silicon fromsilane.

Preferably the method according to the invention is integrated into ageneral method for producing silane and/or hyper-pure silicon.

It is particularly preferred that the method according to the inventionbe integrated into a method for producing silane and/or hyper-puresilicon comprising the following steps:

-   1. Trichlorosilane synthesis on the basis of silicon, silicon    tetrachloride, hydrogen and, if necessary, another chloride source    in a fluidized bed reactor under pressure and subsequent isolation    of the produced trichlorosilane by distillation and recycling of the    unreacted silicon tetrachloride, and, if desired, the unreacted    hydrogen.-   2. Disproportionation of trichlorosilane to silane and silicon    tetrachloride through the intermediate stages of dichlorosilane and    monochlorosilane according to the method according to the invention    on alkaline catalysts, preferably catalysts containing amino groups,    carried out in two apparatuses or in one, and recycling of the    produced silicon provided as a high-boiling component into the first    reaction area.-   3. Further use of the silane of the purity given after the preceding    step, or purifying the silane until the purity required for the    intended purpose is achieved, preferably by distillation,    particularly preferred by distillation under pressure.    -   and, if necessary,-   4. Thermal decomposition of silane to obtain high-purity silicon,    usually above 500° C.

Apart from thermal decomposition on electrically heated high-puritysilicon rods, another suitable method is the thermal decomposition in afluidized bed consisting of hyper-pure silicon particles, particularlywhen the production of solar-grade high-purity silicon is desired. Tothis aim, silane can be mixed with hydrogen and/or inert gases at a molratio of 1:0 to 1:10.

The invention will be further explained by the following examples, whichare provided as illustrations of the invention—but not limiting it tothem.

EXAMPLES Example 1 Comparative Experiment Thermal Drying (Static)

An ion exchanger LEWATIT® MP 62 (BAYER AG) was suspended five times inhot distilled water of approx. 90° C. (ratio of volumes ofcatalyst:water approx. 1:1), each time for a period of 0.5 h, and thewater was removed each time by decantation. The catalyst was then driedat 60° C. in a glass bowl in a drying oven. After 12 h the remainingmoisture contents amounted to 3 weight percent according to the KarlFischer titration.

Example 2 Comparative Experiment Thermal Drying in the Inert Gas Flow

An ion exchanger LEWATIT® MP 62 (BAYER AG) was suspended five times inhot distilled water of approx. 90° C. (ratio of volumes ofcatalyst:water approx. 1:1), each time for a period of 0.5 h, and thewater was removed each time by decantation. The catalyst was thentreated at 70° C. in a reaction pipe in a flow of nitrogen. After 12 hthe remaining moisture contents amounted to 0.15 weight percentaccording to the Karl Fischer titration.

Example 3 Comparative Experiment Thermal Drying in a Vacuum

An ion exchanger LEWATIT® MP 62 (BAYER AG) was suspended five times inhot distilled water of approx. 90° C. (ratio of volumes ofcatalyst:water approx. 1:1), each time for a period of 0.5 h, and thewater was removed each time by decantation. The catalyst was thentreated at 70° C. and 30 hPa in a reaction pipe in a vacuum. After 12 hthe remaining moisture contents amounted to 0.2 weight percent accordingto the Karl Fischer titration.

Example 4

An ion exchanger LEWATIT® MP 62 (BAYER AG) was suspended five times inhot distilled water of approx. 90° C. (ratio of volumes ofcatalyst:water approx. 1:1), each time for a period of 0.5 h, and thewater was removed each time by decantation. This catalyst wastransferred into a reaction pipe with a frit bottom, a supply valve anda discharge valve, in which it was mixed with anhydrous methanol so thatis was completely covered with it. After 5 minutes the discharge valvewas opened and the solvent allowed to drain off, then the valve wasclosed again and the process repeated. Altogether the reaction pipe wasrefilled with fresh solvent five times. The catalyst was then treated at70° C. and 30 hPa in the reaction pipe in a vacuum. After 12 h theremaining moisture contents amounted to 0.18 weight percent according tothe Karl Fischer titration.

Example 5

An ion exchanger LEWATIT® MP 62 (BAYER AG) was suspended five times withhot distilled water of approx. 90° C. (ratio of volumes ofcatalyst:water approx. 1:1), each time for a period of 0.5 h, and thewater was removed each time by decantation. This catalyst wastransferred into a reaction pipe with a frit bottom a supply valve and adischarge valve, in which it was mixed with anhydrous methanol so thatis was completely covered with it. After 5 minutes the discharge valvewas opened and the solvent allowed to drain off, then the valve wasclosed again and the process repeated. Altogether the reaction pipe wasrelined with fresh solvent five times. After the last methanol treatmentthe catalyst was mixed for 0.5 h with a solution of 3 weight percentthionylchloride in n-heptane. The catalyst was then treated at 70° C.and 30 hPa in the reaction pipe in a vacuum. After 12 h the remainingmoisture contents amounted to 0.06 weight percent according to the KarlFischer titration.

Example 6

An ion exchanger LEWATIT® MP 62 (BAYER AG) was suspended five times withhot distilled water of approx. 90° C. (ratio of volumes ofcatalyst:water approx. 1:1), each time for a period of 0.5 h, and thewater was removed each time by decantation. This catalyst wastransferred into a reaction pipe with a frit bottom, a supply valve anda discharge valve, in which it was mixed with anhydrous methanol so thatis was completely covered with it. After 5 minutes the discharge valvewas opened and the solvent allowed to drain off, then the valve wasclosed again and the process repeated. Altogether the reaction pipe wasrefilled with fresh solvent five times. Afterwards the same procedurewas repeated by five treatments with anhydrous n-heptane. The catalystwas then treated at 70° C. and 30 hPa in the reaction pipe in a vacuum.After 12 h the remaining moisture contents amounted to 0.20 weightpercent according to the Karl Fischer titration. The example shows thatthe reduction of the remaining moisture contents in example 5 is to beattributed to the additional chemical drying by means of thionylchlorideand not to the added n-heptane.

Example 7

The catalysts of Examples 2 (comparison) and 4 (according to theinvention) were compared with each other with respect to containedthermally desorptable compounds. To this end, catalysts which werepretreated according to Examples 2 and/or 4, were tempered for 1 h at90° C. and the desorpted compounds frozen out at a low temperature. Thefrozen-out samples were evaluated quantitatively (weighing, ppm based onthe amount of catalyst used) and qualitatively (GC-MS). The catalyst ofExample 4 that was pretreated according to the invention showed a lowercontents in the waste-gas with respect to the substance classes amines,glycol, hydrocarbon (HC) and other trace elements than the catalyst ofExample 2 (see also the table below).

Catalyst Catalyst Example 2 Example 4 Methanol [ppm] 0 4 Amines [ppm] 41 Glycol [ppm] 12 0 HC (C10–C16) [ppm] 163 127 others [ppm] 4 0 Total[ppm] 183 132

1. A method for the manufacture of compounds according to the formula(H)_(x)Si(cl)_(y)  (I), wherein x is 2, 3 or 4, and y is 0, 1 or 2, andx+y=4, by disproportionation of at least one of a hydrochlorosilane ofthe formula(H)_(a)Si(Cl)_(b)  (II), wherein a is 1, 2 or 3, and b is 1, 2 or 3, anda+b=4 and b>y, and a mixture of such hydrochlorosilanes in the presenceof a catalyst, wherein the catalyst prior to use a) is washed withhyper-pure water in one or several steps, b) is transferred in awater-moist state into the reactor where the disproportionation shall becarded out, and c) is at least one of treated with boiling methanol andrinsed with methanol in the reactor, and d) the methanol is removed fromthe catalyst by evacuation and/or by shipping with inert gas.
 2. Amethod according to claim 1, wherein the compound in formula I is silaneand the hydrochlorosilane in formula II is at least one oftrichlorosilane, dichlorosilane and a mixture thereof.
 3. A methodaccording to claim 1, wherein the compound in formula I isdichlorosilane and the hydrochlorosilane in formula II istrichlorosilane.
 4. A method according to claim 1, wherein thedisproportionation is carried out according to the principle of reactivedistillation.
 5. A method according to claim 1, wherein the washing ofthe catalyst is carried out with hyper-pure water (step a) attemperatures from 60 to 90° C.
 6. A method according to claim 1, whereinthe treatment of the catalyst in the reactor is carried out withmethanol (step c) by heating the methanol at the lower-most end of thereactor until it boils, and allowing it to condense at the upper end ofthe reactor.
 7. A method according to claim 1, wherein the treatment ofthe catalyst with methanol (step c) is carried out by rinsing withanhydrous methanol.
 8. A method according to claim 1, wherein thetreatment of the catalyst with methanol in the reactor (step c) iscarried out at least two times with freshly introduced methanol.
 9. Amethod according to claim 1, wherein the ratio of volumes of methanol tocatalyst during the treatment of the catalyst in the reactor withmethanol (step c) is 0.1:1 to 1:0.3.
 10. A method according to claim 1,wherein after the removal of the methanol from the catalyst (step d),the catalyst is subjected to a chemical post-drying procedure.